IMMOBILIZATION OF ALKOXYLATED PHOSPHINE-LIGANDS AND THEIR RH COMPLEXES TO A SILICA SURFACE-COATED WITH AN ORGANIC MONOLAYER OR MULTILAYER

A new methodology for the immobilization of homogeneous catalysts on a modified silica surface is suggested. Phosphine ligands with polar su bstituted groups as well as their rhodium carbonyl complexes are attac hed to modified silica substrates. The modified silica substrates were prepared by either depositing a crosslinked poly(hydroxyethyl methacr ylate) network on the surface of porous silica, or by transforming the surface-pendant hydroxyl groups of silica into [3-(siloxysilyl)propyl ]ethylenediamine groups. They are denoted as P(HEMA-MBAM)/SiO2 and SSP EDA/SiO2, respectively. The following phosphine ligands were attached to the modified silica: (1) alpha,omega-bis(diphenylphosphino)-poly(et hylene glycol) (DPP-PEG); (2) 2-methoxy-methylenephenyl-diphenylphosph ine (MPDPP); and (3) 1,2-bis oxyacetylphenyl)-phenyl-diphenyl)]phosphi no)ethane (MAPDPPE). The FT-IR investigations of these immobilized cat alysts revealed hydrogen bonding between the surface-pendant hydroxyl or amino groups and the oxygen atoms of the phosphine ligands. The SEM and EDS surface analyses showed that the phosphine rhodium carbonyl c omplexes were uniformly distributed over the polymer coated silica. Hy droformylation reactions of several olefins were employed for the asse ssment of the efficiency of immobilization, and both hydrophobic (cycl ohexane) and hydrophilic (water) reaction media have been employed. Th e time dependence of the hydroformylation of 2-(+/-)-ethylhexyl acryla te (EHA) in a hydrophobic medium indicated that the modified silica su rfaces are more compatible with the hydrophobic reactant molecules tha n the pure silica surface, and that SSPEDA/SiO2 is more effective than P(HEMA-MBAM)/SiO2. The much higher hydroformylation rate for sodium 1 0-undecenoate (UDNa) than for I-vinylimidazole when water was used as reaction medium is due to the higher ability of the former surface act ive molecules to be adsorbed on the catalyst surface. Successive reuse s of the immobilized catalysts in the hydroformylation of ethyl 10-und ecenoate in cyclohexane have demonstrated that MAPDPPE provides strong er immobilization than MPDPP.